A conventionally known articulated measuring apparatus that measures a complicated three-dimensional shape of an object to be measured includes a flexible articulated measuring arm. For example, Japanese Patent Laid-Open No. 2004-264135 (pp. 8-14, FIG. 1) discloses such an articulated measuring apparatus. With the conventional articulated measuring apparatus, a user moves the measuring arm to bring a probe mounted to a tip of the measuring arm close to an arbitrary measurement point on the object to be measured and measure a position or a size of the object to be measured. The articulated coordinate measuring apparatus includes therein an angle sensor (for example, a rotary encoder) that measures a rotation angle of each joint of the measuring arm. Then, a space coordinate of a tip of the probe is calculated based on the rotation angle of each joint of the measuring arm and a design length between joints or between a joint and a probe. Specifically, a calculation process is performed of converting a parameter indicating the rotation angle obtained from the angle sensor and a parameter indicating the length between joints or between a joint and a probe into space coordinates, that is, of converting a coordinate system of the measuring arm into a coordinate system of the object to be measured.
When performing measurement using the above-described articulated measuring apparatus, the user sometimes operates the measuring arm with a strong force. For example, the user sometimes strongly pulls, bends, or shortens the measuring arm. When stress is thus applied to the measuring arm, errors may occur in parameters read from each sensor of the measuring arm.
The articulated measuring apparatus requires measurement of a complicated three-dimensional shape with high accuracy. However, the errors occurring in each parameter of the measuring arm of the articulated measuring apparatus as described above make difficult the measurement with high accuracy. Then, conventionally, a method has been proposed of calibrating each parameter of a measuring arm using an artifact (standard unit) to reduce parameter errors. For example, Japanese Patent Laid-Open No. 2005-157784 (pp. 12-26) discloses such a method.
However, in the conventional method, all parameters of the measuring arm are calibrated. The articulated measuring arm has many joints, and thus many parameters are to be calibrated. Calibration work of parameters of a measuring arm is essential preparation work for measurement with high accuracy, but calibration of all the parameters takes much labor and time. It is certain that sufficient time needs to be taken to perform calibration work of all the parameters in factory shipment, apparatus installation, or the like. However, a user who daily performs measurement needs to daily perform calibration work of parameters, and reductions in labor and time for daily calibration work is desired.